Electron device



April 29, 1941;

F. H. NICOLL ELECTRON DEVICE Filed April 6,' 1938.

lNVENTOR FREDERICK H NICOLL ATTORNEY Patented Apr. 29, 1941 ELECTRON DEVICE Hayes, Middlesex, Great Britain England,

a company of Application April 6, 1938, Serial No. 200,284 In Great Britain April 1'7, 1937 4 Claims.

This invention relates to focusing systems for use in electric discharge devices and has particular reference to electron lenses employing electrodes to which voltages of selected ratios are applied.

Cathode ray tubes employing electron lenses consisting of, for example, two or more cylindrical elements to which differing voltages are applied are well known and electron focusing by means of electro-magnetic coils is also well known. In the case of an electron lens employing two cylindrical focusing electrodes, it is known that a focal length of the order of a few tube diameters is obtained with voltage ratios 1 1 between the electrodes of between 2 and I2, V2 and V1 being the potentials of the cylindrical electrodes. The focal length decreases as the focusing ratio increases and this can bedemonstrated by passing a parallel beam of electrons through two co-axial cylinders, one of which is maintained at a potential V2 which is higher than the potential V1 of the other cylinder Where the ratio is between 2 and 15. The beam will come to a focus in the field free space of the cylinder at the voltage V2. Such a form of electron lens is normally used in cathode ray tubes for focusing a parallel or diverging beam of electrons to a point on the fluorescent screen. According to the present invention, in a method of operating an electric discharge device containing an electrode system for focusing a beam of electrons or ions, in order to obtain a high beam velocity, the ratio of voltages applied to the electrodes forming an electron lens is such that said beam is brought to an intermediate focus within or in the neighborhood of the lens field and is further focused at a point which is substantially free from the field set up by said electron lens. The electron beam may be emitted by a cathode or may emanate from a photoelectric surface or an object may be placed in the path of the beam so that an image of that object is formed at a desired point. The further. point at which the beam is focused or the point at which the image of an object if formed may coincide with the surface of the fluorescent screen in the case of a cathode ray tube or the anti-cathode in the case of an X-ray tube. The ratio of the voltages applied to two elements of an electron lens may be of the order of :1 or the ratio may be greater than 70:1. Thus, for example, the ratio may be of the order of to 1 or of several thousand to 1, the selection of values being dependent upon the geometrical form of the elements of the lens.

In order that the invention may be more clearly understood and readily carried into effect, an electron lens system operated in accordance therewith will now be described by way of example with reference to the drawing, in which:

Figure 1 shows an electron lens for explaining my invention;

Figure 2 illustrates the form of focusing field in a lens according to the invention, and

Figure 3 shows such a lens system forming part of an X-ray tube.

The present applicant has found that a parallel electron beam can be brought to a focus in the field free space of the second electrode at a focusing ratio very much higher than those previously mentioned. As pointed out above, the focal length decreases with an increasing focusing ratio. Thus, the distance F from the centre of the lens comprising electrodes l and 2 in Figure 1, to the focus, decreases as the voltage V2 applied to the electrode 2 increases relatively to the voltage V1 applied to the electrode I.

If the focusing ratio continues to increase, the focal length F finally becomes zero at about a focus ratio of '70, and at this point the beam crosses over just at the centre of the lens.

Referring now to Figure 2 of the drawing, for focusing ratios above 10, the beam actually crosses over at the point X in the converging field of the first half of the lens and is further converged .by the remainder of the converging field and then passes through the weakly diverging field of the second half of the lens to come to a focus in the field free space at the point W on screen Y.

A focusing system as described with reference to Figure 2 may be employed with advantage in cathode ray X-ray tubes and ion focusing arrangements in which it is desired to accelerate the beam 100 to 10000 times, and at the same time to bring the beam to a focus at a point some distance from the source. Thus, Figure 3 shows diagrammatically part of the electrode system of an X-ray tube 14 to which the invention is applied wherein the electron beam from a cathode 5 and an initial focusing cylinder 6 is accelerated by the first and second anodes I and 8, the anode 1 having a voltage of 1000 volts applied to it, such a voltage providing a convenient accelerating potential and also a means of varying the focusing. The second anode 8 has 100,000 volts applied to it, and the beam is brought to a focus on the anti-cathode 9. Leads in, H, I2 and I3 are provided for supplying potentials to the electrodes 6, l, and 8 respectively. The focusing ratio may be between 100 and several thousand but it will depend on the geometrical form of the electrodes of the system. Thus, as the diameter of the axially 'symmetrical electrodes is reduced the voltage ratio for focusing needed to obtain the same result will be reduced. Again, a double apertured lens of a given diameter will require a slightly higher voltage ratio for focusing than a two cylinder lens of the same diameter assuming the relative distances of the object and image are maintained constant.

The beam of electrons which is focused by a lens system operated in accordance with the invention may emanate directly from a hot cathode or it may emanate from a photo-electric surface upon which a light image is incident, the electron image so produced being focused upon a fluorescent screen or upon a mosaic screen in a transmitting tube. Again it may be desired to reproduce upon a screen an image of an object placed within a beam of electrons as, for example, a grid placed in the beam in the first cylinder of the lens system. Thus, referring to Fig; 3 of the drawings, it will be seen that the image focused on the screen 9 is that of the first crossover point near the cathode 5 and this image is produced by the method described which results in an intermediate cross-over in the lens field". The intermediate cross-over point represents an intermediate image of the first cross-over point and if the voltage of the anode 8 is gradually reduced while the voltage applied to the anode! is maintained constant, then the image on the screen 9 would slowly disappear and the intermediate crossover point would gradually move towards the screen 9 and would finally be focused upon it. The focused image may, therefore, be

that of a grid placed within the first cylinder; In such a case, as the voltage of the anode 8 is slowly raised above that of the anode I, a sharply focused image of the grid would appear at'a low focusing ratio, this image disappearing as the voltage applied to the anode 8 continues to increase, until at a very much higher ratio a second sharp image of the grid would be obtained. It will be understood that the intermediate image will be inverted but the final image will be erect, and any desired object other than a grid may be placed within the first cylinder.

It will be further appreciated that the anode 9 may be replaced by a fluorescent screen and the electrode 6 supplied with television signals and deflecting means well known in the art, provided intermediate the electrode 8 and screen to provide a television projection tube, since such tubes. as well known, require high potentials to produce the required brilliancy.

Having described myinvention, what I claim is:

1. In a cathode ray tube wherein is provided an electron emitter, a target electrode and a plurality of substantially tubular accelerating electrodes longitudinally spaced from each other and each in register with and intermediate the electron emitter and the target electrode, the method of focussing which comprises releasing electrons from the emitter, producing a first electrostatic field by applying positive voltage to one of the plurality of accelerating electrodes relative to the cathode to move the released electrons toward the target in substantially beam formation, producing a second electrostatic field intermediate the first field and target by applying a positive voltage to a second accelerating electrode relative to the, cathode, said last-named field being substantially contiguous with the first field to increase the rate of motion of the electrons toward the target, adjusting the magnitude of the two electrostatic fields relative to each other by adjusting the voltage applied to values such that crossover of electrons of the beam takes place within the first accelerating electrode at a point located closely adjacent tos the terminating plane thereof nearest. the target, and then focussing the electrons passing beyond the second-named accelerating electrode upon the target.

2. In a cathode ray tube system an electron emitting cathode,-an initial focusing electrode positioned in register with said cathode, a first cylindrical anode electrode adjacent to and in register with said initial focusing electrode, a second cylindrical anode electrode adjacent to and in register with the first cylindrical anode electrode, a target electrode positioned beyond the second cylindrical anode electrode and in register therewith, and means including sources of positive potentials to produce cross-over of electrons from the cathode within the space enclosed by the firstcylindrical anode electrode immediately adjacent the second anode electrode and to focus said electrons upon the targe electrode.

3. In a cathode ray tube system an electron emitting cathode, an initial focusing electrode positioned in register with said cathode, a first cylindrical anode electrode adjacent to and in register with said initial focusing electrode, a second cylindrical anode electrode adjacent to and in register with the first cylindrical anode electrode, a target electrode positioned beyond the second cylindricalanode electrode and in regis ter therewith, and means for applying different positive potentials toeach of said electrodes and for maintaining the ratio of the potential of the second cylindrical anode electrode to the first cylindrical anode electrode of not less than '70.

4. In a cathode ray tube system an electron emitting cathode, an initial focusing electrode positioned in register with said cathode, a first cylindrical anode electrode adjacent to and in register with said initial focusing electrode, a

second cylindrical anode electrode adjacent, to

and in register with the first cylindrical anode electrode, a target electrode positioned beyond the second cylindrical anode electrode and in register therewith, and means for applying different positive potentials to each of said electrodes and for producing cross-over of electrons from the 4 cathode within the field of the first cylindrical anode electrode contiguous to the field of the second cylindrical anode electrode and for'subsequentlyfocusing the electrons upon the target electrode.

FREDERICK HERIWES NICOLL. 

